Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 35

through which the atom diffuses. As a consequence, in the polycrystalline solid, as
the surface area (or grain boundary area) to volume ratio is usually very small, the
grain boundaries and surfaces become important in particular phenomena such as
sintering, oxidation etc. It is also obvious that when the grain size is finer and the
temperature is lower which follows from the lower activation energy that makes it
less sensitive to temperature change, grain boundary diffusion becomes more com-
petitive (Smallman and Bishop 1999 ).
Pressure has little or no effect on the rate of diffusion. Diffusion usually takes
place by the motion of individual molecules in polymeric materials where strong
covalent bonds exist within individual molecules and where the molecules are
bound together by weak secondary bonds (Smallman and Ngan 2007 ). In general,
the larger molecules possess higher activation energy, as an example, the activa-
tion energy for diffusion of CH 3 molecules in nylon is 6.3 kJ/mole whereas C 4 H 9
molecule is 8.8 kJ/mole.
2.6.2 Fickian and Non-Fickian Diffusion in Polymeric
Materials
By a concentration-dependent form of Fick's Law with constant boundary condi-
tions, the diffusion behaviour of many polymers cannot be described sufficiently
(Crank 1979 ). This is particularly true when the penetrant affects extensive swell-
ing of the polymer. This case with so-called glassy polymers exhibits “anomalous”
or “non-fickian” behaviour. Diffusion is generally Fickian in rubbery polymers.
The essential distinction between glassy polymers and rubbery polymers is that
the rubbery state responds quickly to changes in their condition. As an example,
any change in temperature causes an almost immediate change to a new equilib-
rium volume. On the other hand, the properties of a glassy polymer tend to be
time-dependent, if this type of polymer has been stretched, the stress may be slow
to decay. It is considered that the deviations from Fickian behaviour is associated
with the finite rates at which with the response to the sorption or desorption of
penetrant molecules, the polymer structure can change. It is also described that
anomalous effects might be related directly to the influence of the changing poly-
mer structure on solubility and diffusional mobility (Crank 1979 ). It is also pos-
sible that as diffusion proceeds, inconsistency may result from the internal stresses
exerted by one part of the medium on another.
Usually polymers possess a wide spectrum of relaxation times which is asso-
ciated with structural changes. All of them decrease as temperature or penetrant
concentration is increased and also when the motion of the polymer segments
enhanced. The change from the glassy to the rubbery state occurs at the glass tran-
sition temperature at a given concentration. As a result, the sorption process is
influenced by those segmental motions that can occur approximately at the same
rate or slower than the motivating diffusion process.